Handover performed in consideration of uplink/downlink component carrier setup

ABSTRACT

The present invention relates to a wireless communication system which supports carrier aggregation. More particularly, the present invention relates to a method and to an apparatus for enabling a user equipment to perform a handover in a wireless communication system which supports carrier aggregation. The method for performing a handover comprises the steps of: transmitting a measurement report on a target cell to a serving cell; receiving, from the serving cell, a message containing a signature route sequence index, cyclic shift parameters, and information related to the component carrier of the target cell; confirming contention-based signatures generated on the basis of the signature route sequence index and cyclic shift parameters; and transmitting one of said contention-based signatures to the target cell for random access, via one or more component carriers, on the basis of said information related to the component carrier. The present invention also relates to an apparatus for the method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/326,058, filed on Jul. 8, 2014, now U.S. Pat. No. 9,338,705, which isa continuation of U.S. patent application Ser. No. 13/256,200, filed onSep. 12, 2011, now U.S. Pat. No. 8,811,350, which is the National Stagefiling under 35 U.S.C. 371 of International Application No.PCT/KR2010/001587, filed on Mar. 15, 2010, which claims the benefit ofU.S. Provisional Application No. 61/265,728, filed on Dec. 1, 2009 and61/159,862, filed on Mar. 13, 2009, the contents of which are all herebyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system. Moreparticularly, the present invention relates to a method and an apparatusfor performing a handover.

BACKGROUND ART

Wireless communication systems are widely developed to provide a variouskinds of communication services such as audio or data service. Ingeneral, a wireless communication system is a multiple access systemcapable of supporting communications with multiple users by sharingavailable system resources (bandwidths, transmission power, etc.).Examples of the multiple access system include a CDMA (Code DivisionMultiple Access) system, FDMA (Frequency Division Multiple Access)system, TDMA (Time Division Multiple Access) system, OFDMA (OrthogonalFrequency Division Multiple Access) system, SC-FDMA (Single CarrierFrequency Division Multiple Access) system, MC-FDMA (Multi-CarrierFrequency Division Multiple Access) system, etc.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and an apparatus for performing a handover in a wirelesscommunication system. Specifically, the present invention provides amethod and an apparatus for performing a handover in a wirelesscommunication system which supports carrier aggregation.

It will be appreciated by persons skilled in the art that the objectsthat can be achieved with the present invention are not limited to whathave been particularly described hereinabove and the above and otherobjects that the present invention can achieve will be more clearlyunderstood from the following detailed description.

Technical Solution

In accordance with one aspect of the present invention, there isprovided a method for enabling a user equipment to perform a handover ina wireless mobile communication system which supports carrieraggregation, the method including the steps of: transmitting ameasurement report on a target cell to a serving cell; receiving, fromthe serving cell, a message containing a signature route sequence index,cyclic shift parameters, and information related to the componentcarrier of the target cell; recognizing contention-based signaturesgenerated on the basis of the signature route sequence index and cyclicshift parameters; and transmitting one of the contention-basedsignatures to the target cell for random access, via one or morecomponent carriers, on the basis of the information related to thecomponent carrier.

In accordance with another aspect of the present invention, there isprovided a user equipment including: an RF (Radio Frequency) module forreceiving, from a source base station, a message containing a signatureroute sequence index, cyclic shift parameters, and information relatedto the component carrier of a target cell, and for transmitting a randomaccess signature to the target base station; and a processor processingthe message containing the signature route sequence index, the cyclicshift parameters, and the information related to the component carrierof the target cell, and preparing the random access signature based onthe signature route sequence index and the cyclic shift parameters,wherein the random access signature is transmitted to the target basestation via a component carrier identified by the information related tothe component carrier of the target cell.

The information related to the component carrier may include informationon allocation of component carriers assigned by the target cell to theuser equipment. The information on allocation of component carriers mayinclude index information related to an uplink component carrierperforming the random access. The index information may include an indexof a downlink component carrier linked with the component carrier whichperforms the random access.

Advantageous Effects

According to embodiments of the present invention, a handover can beefficiently performed in a wireless communication system. Specifically,a handover can be efficiently carried out in a wireless communicationsystem which supports carrier aggregation.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 shows a network structure of an E-UMTS (Evolved Universal MobileTelecommunication System);

FIG. 2 shows structures of an E-UTRAN (Evolved Universal TerrestrialRadio Access Network) and a gateway;

FIGS. 3 and 4 show user/control plane protocols with respect to anE-UMTS;

FIG. 5 shows a structure of a radio frame used in an E-UMTS;

FIG. 6 illustrates a general handover process of a 3GPP (3^(rd)Generation Partnership Project) LTE (Long Term Evolution) system;

FIG. 7 shows an example of performing communication in a carrieraggregation state;

FIG. 8 illustrates a general handover process according to an embodimentof the present invention;

FIG. 9 illustrates a handover process according to an embodiment of thepresent invention; and

FIG. 10 is a block diagram of a mobile station according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Configurations, operations and other characteristics of the presentinvention will be easily understood according to embodiments of thepresent invention, described with reference to the attached drawings.Though the following embodiments will describe a case in which technicalcharacteristics of the present invention are applied to a 3GPP system,the embodiments are exemplary and the present invention is not limitedthereto.

FIG. 1 shows a network structure of an E-UMTS. The E-UMTS is alsoreferred to as a LTE system. A communication network is arranged in awide range and provides various communication services such as audio andpacket data service.

Referring to FIG. 1, an E-UMTS network includes an E-UTRAN (EvolvedUniversal Terrestrial Radio Access Network), an EPC (Evolved PacketCore), and one or more user equipments (UE). The E-UTRAN may include oneor more base stations (eNB) 20. The one or more UEs 10 may be located inone cell. One or more E-UTRAN mobility management entity/systemarchitecture evolution (MME/SAE) gateways 30 may be located at the endof the network and connected to an external network. In thespecification, a downlink means transmission from the base station 20 tothe UE 10 and an uplink means transmission from the UE 10 to the basestation 20.

The UE 10 is a communication device carried by a user and may bereferred to as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), or a radio device. Each base station 20 is a fixed stationcommunicating with the UE 10 and may be referred to as an access point(AP). The base station 20 provides end points of a user plane and acontrol plane to the UE 10. One base station 20 may be located in eachcell. An interface for transmitting user traffic or control traffic maybe used between the base stations 20. Each MME/SAE gateway 30 providesend points of session and mobility management function to the UE 10. Thebase station 20 and the MME/SAE gateway 30 can be connected to eachother through an S1 interface.

MME provides various functions including distribution of a pagingmessage to the base stations 20, security control, idle state mobilitycontrol, SAE bearer control, and encryption of non-access stratum (NAS)layer signaling and integrity protection. An SAE gateway host providesvarious functions including completion of a plane packet and user planeswitching for supporting mobility of the UE 10. The MME/SAE gateway 30is simply referred to as a gateway in the specification. However, theMME/SAE gateway 30 includes both MME and SAE gateways.

A plurality of nodes may be connected through S1 interfaces between thegateways 30 and the base stations 20. The base stations 20 may beconnected to each other through an X2 interface and neighbor basestations may have a mesh network structure with the X2 interface.

FIG. 2 shows structures of a general E-UTRAN and the general gateway 30.Referring to FIG. 2, the base station 20 can execute functions such asselection of the gateway 30, routing to the gateway during activation ofradio resource control (RRC), scheduling and transmission of a pagingmessage, scheduling and transmission of broadcast channel (BCCH)information, dynamic resource allocation for the UE 10 on both uplinkand downlink, configuration and preparation of base station measurement,radio bearer control, radio admission control (RAC), and connectionmobility control. The gateway 30 can perform functions such as pagingtransmission, LTE_IDLE state management, user plane encryption, systemarchitecture evolution bearer control, encryption of NAS layer signalingand integrity protection.

FIGS. 3 and 4 show user-plane protocol and control-plane protocol stacksfor an E-UMTS. Referring to FIGS. 3 and 4, protocol layers can bedivided into a first layer L1, a second layer L2, and a third layer L3on the basis of lower three layers of the open system interconnection(OSI) standard model known in communication system technologies.

A physical layer PHY corresponding to the first layer L1 providesinformation transmission to an upper layer using a physical channel. Thephysical layer is liked to a medium access control (MAC) layer locatedat an upper level through a transmission channel, and data istransmitted between the physical layer and the MAC layer through thetransmission channel. Data is transmitted between a physical layer of atransmitter and a physical layer of a receiver through a physicalchannel.

An MAC layer corresponding to the second layer L2 provides a service toa radio link control (RLC) layer corresponding to an upper layer througha logical channel. The RLC layer of the second layer L2 supportsreliable data transmission. When the MAC layer performs an RLC function,the RLC layer is included in the MAC layer as a functional block. A PDCP(Packet Data Convergence Protocol) layer of the second layer L2 performsa header compression function. The header compression functionefficiently transmits an Internet protocol (IO) packet such as IPv4 orIPv6 through a radio interface having a relatively narrow bandwidth.

A radio resource control (RRC) layer located at the lowest level of thethird layer L3 is defined for a control plane only. The RRC layercontrols a logical channel, a transmission channel and a physicalchannel with respect to setup, re-setup and cancellation of radiobearers (RBs). A radio bearer (RB) means a service provided by thesecond layer L2 for data transmission between the UE 10 and the E-UTRAN.

Referring to FIG. 3, the RLC layer and the MAC layer are finished in thebase station 20 and can perform functions such as scheduling, automatedretransmission request (ARQ) and hybrid automated retransmission request(HARQ). The PDCP layer is finished in the base station 20 and canexecute functions such as header compression, integrity protection andencryption.

Referring to FIG. 4, the RLC layer and the MAC layer are completed inthe base station 20 and perform the same function as those in thecontrol plane. As shown in FIG. 3, the RRC layer is finished in the basestation 20 and can perform functions such as broadcasting, paging, RRCconnection management, radio bearer control, mobility function, and UEmeasurement report and control. As shown in FIG. 2(c), a NAS controlprotocol is finished in MME of the gateway 30 and can execute functionssuch as SAE bearer management, authentication, LTE_IDLE mobilityhandling, paging transmission in LTE_IDLE state, and security controlfor signaling between the gateway and the UE 10.

The NAS control protocol can use three states. A LTE-DETACHED state isused when RRC entity is not present. A LTE_IDLE state stores minimum UEinformation and is used when RRC connection is not present. A LTE_ACTIVEstate is used when RRC state is set up. The RRC state is divided intoRRC_IDLE and RRC_CONNECTED states.

In the RRC_IDLE state, the UE 10 perform discontinuous receiving (DRX)set by NAS using ID uniquely allocated thereto in a tracking region.That is, the UE 10 can receive broadcast of system information andpaging information by monitoring a paging signal on a specific pagingoccasion for each UE-specific paging DRX cycle. In the RRC_IDLE state,no RRC context is stored in the base station.

In the RRC_CONNECTED state, the UE 10 can transmit/receive data to/fromthe base station using E-UTRAN RRC connection and context in E-UTRAN.Furthermore, the UE 10 can report channel quality information andfeedback information to the base station. In the RRC_CONNECTED state,the E-UTRAN is aware of the cell to which the UE 10 belongs.Accordingly, the corresponding network can transmit and/or receive datato and/or from the UE 10, control mobility of the UE, such as ahandover, and perform cell measurement with respect to neighboringcells.

FIG. 5 shows a structure of a radio frame used in an E-UMTS.

Referring to FIG. 5, the E-UMTS uses a radio frame of 10 ms. One radioframe includes ten subframes. One subframe has two continuous slots. Thelength of one slot is 0.5 ms. Furthermore, one subframe is composed of aplurality of symbols (for example, OFDM symbols, SC-FDMA symbols, etc.).One subframe is composed of a plurality of resource blocks, and oneresource block includes a plurality of symbols and a plurality ofsubcarriers. Some (for example, a first symbol) of the plurality ofsymbols constituting one subframe can be used to transmit L1/L2 controlinformation. A physical channel (for example, PDCCH (Physical DownlinkControl Channel)) transmitting the L1/L2 control information is composedof subframes on the time domain and subcarriers on the frequency domain.

FIG. 6 illustrates a handover process defined in the LTE. FIG. 6 shows acase in which MME and a serving gateway are not changed. The handoverprocess is described below in detail with reference to 3GPP TS(Technical Specification) 36.300.

Step 0: UE context in a source base station eNB includes information onconnection setup or roaming restriction set in the event of latest TAupdate.

Step 1: The source base station sets up an UE measurement process basedon area restriction information. The measurement provided by the sourcebase station can assist control of connection mobility of an UE.

Step 2: The UE is triggered to transmit measurement report according toa rule set by system information, etc.

Step 3: The source base station determines whether or not to handoverthe UE on the basis of the measurement report and RRM (Radio ResourceManagement) information.

Step 4: The source base station transmits information for a handover toa target base station through a handover request message. Theinformation required for the handover includes UE X2 signaling contextreference, UE S1 EPC signaling context reference, target cell ID, RRCcontext including UE identifier (for example, Cell Radio NetworkTemporary Identifier; C-RMTI) in the source base station, etc.

Step 6: The target base station prepares L1/L2 and handover andtransmits a handover request acknowledge (ACK) message to the sourcebase station. The handover request ACK message includes a transparentcontainer (RRC message) transmitted to the UE to perform the handover.The container includes new C-RNTI and a security algorithm identifier ofthe target base station. In addition, the container may additionallyinclude an access parameter and an additional parameter such as SIB.Furthermore, the target base station can divide RA signatures into anon-contention based RA signature set (referred to as group 1hereinafter) and a contention based RA signature set (referred to asgroup 2 hereinafter), select one of signatures of group 1 and inform theUE of the selected signature. That is, the container may further includeinformation on a dedicated RA signature. Moreover, the container mayinclude information on an RACH slot duration for which the dedicated RAsignature will be used.

Step 7: The source base station generates an RRC message (for example,RRC Connection Reconfiguration message) having mobility controlinformation on the UE and transmits the RRC message to the UE in orderto perform the handover. The RRC connection reconfiguration messageincludes parameters required for the handover (for example, new C-RNTIand the security algorithm identifier of the target base station, andinformation on the dedicated RACH signature and target base station SIBwhich are optional) and instructs the handover to be performed.

The source base station transmits a SN (serial number) status transfermessage to the target base station so as to transfer uplink PDCP SNreception status and transfer downlink PDCP SN transmission status.

Step 8: The UE attempts to access a target cell using a RACH processafter receiving the RRC connection message. RACH is performed on anon-contention basis if a dedicated RACH preamble is allocated andcarried out on a contention basis if not.

Step 9: The network performs uplink allocation and timing adjustment.

Step 10: When the UE successfully accesses the target cell, the UEtransmits RRC Connection Reconfiguration Complete message (C-RNTI) toconfirm the handover and transmits an uplink buffer status report tothereby inform the target base station that the handover process iscompleted. The target base station confirms C-RNTI received through ahandover confirmation message and starts to transmit data to the UE.

Step 11: The target base station transmits a path switch message to theMME so as to indicate that the UE has changed the cell.

Step 12: The MME transmits a user plane update request message to aserving gateway.

Step 13: The serving gateway switches a downlink data path to thetarget. The serving gateway transmits an end marker packet to the sourcebase station through the existing path, and then cancels user plane/TNLresources for the source base station.

Step 14: The serving gateway transmits a user plane update responsemessage to the MME.

Step 15: The MME responds to the path switch message using a path switchACK message.

Step 16: The target base station transmits a UE context release messageto inform the source base station that the handover has beensuccessfully completed and triggers resource release.

Step 17: Upon reception of the UE context release message, the sourcebase station releases user plane related resources which are associatedwith UE context.

FIG. 7 illustrates an example of communication performed in a carrieraggregation state. FIG. 7 can correspond to an example of communicationin an LTE-A (Advance) system. The LTE-A system uses carrier aggregationor bandwidth aggregation which aggregates a plurality of uplink/downlinkfrequency blocks to use wider uplink/downlink bandwidths in order to usea wider frequency band. Each frequency block is transmitted using acomponent carrier (CC). The CC may mean a frequency block for carrieraggregation or a center carrier of the frequency block according tocontext, and the frequency block and the center carrier are usedtogether.

Referring to FIG. 7, five 20 MHz CCs can be aggregated inuplink/downlink to support a bandwidth of 100 MHz. The CCs may becontiguous or non-contiguous in the frequency domain. The radio framestructure illustrated in FIG. 5 can be also applied to a case usingmultiple component carriers. FIG. 7 shows a case in which the UL CCs andDL CCs have the same bandwidth and are symmetrical for convenience.However, the bandwidths of the UL CCs and DL CCs can be independentlydetermined. For example, the UL CCs can have bandwidths of 5 MHz (ULCC0)+20 MHz (UL CC1)+20 MHz (UL CC2)+20 MHz (UL CC3)+5 MHz (UL CC4).Asymmetrical carrier aggregation in which the number of UL CCs isdifferent from the number of DL CCs can be used. The asymmetricalcarrier aggregation may be generated due to restriction on availablefrequency band or may be artificially constructed according to networksetup. Furthermore, though FIG. 7 shows that an uplink signal and adownlink signal are transmitted through one-to-one mapped CCs, thesignals may be transmitted through different CCs according network setupor signal type. For example, a CC transmitting a scheduling command maybe different from a CC transmitting data according to the schedulingcommand. Furthermore, uplink/downlink control information may betransmitted through specific UL/DL CCs regardless of whether or not CCsare mapped.

Even when the overall band of a system is composed of N CCs, a frequencyband that a specific UE can receive may be limited to M (<N) CCs.Various parameters for carrier aggregation can be set according to acell-specific, UE group-specific or UE-specific method. Accordingly,when N CCs are presented in a cell, a UE may receive a PDSCH (PhysicalDownlink Shared Channel) through all the N CCs. However, a base stationmay limit the number of CCs through which the UE can receive the PDSCHto M (M<N) in a semi-static manner. Although the following descriptionis made on the assumption that embodiments of the present invention areapplied to N CCs, it is apparent that it is also possible to divide N(or M) CCs allocated to a UE into L CC groups and apply the embodimentsof the present invention to each CC group.

A process of enabling a UE to initially access a serving cell will nowbe described.

Step 1: The UE performs cell search on a frequency raster of 100 kHz or300 kHz.

Step 2: When a SCH (Synchronization channel) is detected from one ofaggregated DL CCs, the UE receives a PBCH (Physical Broadcast Channel)in the DL CC. After the PBCH is received, a DL BW (Downlink Bandwidth),the number of Tx antennas, and PHICH (Physical Hybrid ARQ IndicationChannel) setup are obtained. In addition, the UE acquires systeminformation (SI-2) through the DL CC from which the SCH has beendetected. For example, UL EARFCN (E-UTRA Absolute Radio FrequencyChannel Number), UL BW and configuration of various physical channelsare obtained. Furthermore, upon reception of the PBCH and SI-2, the UEacquires information on an UL CC linked with the DL CC. In case of acarrier aggregation system, it is desirable to receive systeminformation (that is, aggregated CC configuration employed by a cell) onDL and/or UL setup through a single CC during an initial access processsuch that cell search complexity does not increase according to systembandwidth.

Step 3: After cell search and reception of the PBCH, the UE alignsuplink timing and performs random access to obtain UE ID. To achievethis, the UE transmits RACH preamble (that is, signature) on the UL CCon the basis of PRACH configuration of system information receivedthrough the DL CC.

Step 4: The UE receives an RACH response message and transmits an RACHmessage (MSG) 3 to a base station.

Step 5: The base station receives the RACH MSG 3 and transmits RACH MSG4 to the UE.

Step 6: After collision has been overcome, the UE receives UE-specificor UE-common CC allocation information (that is, information onallocated UL/DL CCs) from the base station. CC allocation informationmay be semi-statically transmitted through RRC signaling or dynamicallytransmitted through L1/L2 signaling (for example, PDCCH).

UE-specific CCs can be allocated to an arbitrary UE through initialaccess to a serving cell and UE-specific RRC signaling and/orUE-specific L1/L2 control signaling in the serving cell according to theabove method. Alternatively, UE-common CCs can be allocated to anarbitrary UE through cell (or eNB)-specific RRC signaling and/orUE-common L1/L2 control signaling (for example, PDCCH). UE-specific CCallocation information may include information for allocating one ormore DL CCs and/or one or more UL CCs in different forms according to UEcapability. Furthermore, the UE-specific CC allocation information mayinclude information for UE-specifically overriding fundamental CCconfiguration information (for example, CC index information of activeDL CCs/UL CCs for scheduling, DL CC-UL CC linkage information may beadded according to circumstances) of a cell/base station. For example,if fundamental cell configuration has a symmetrical form in which thenumber of DL CCs equals to the number of UL CCs and the DL CCs and theUL CCs are linked one to one, each UE can be informed of new DL CC-UL CClinkage information such that the UE is allowed to perform asymmetricalcarrier aggregation.

Embodiment 1-1: Handover in a Carrier Aggregations State

FIGS. 8 and 9 illustrate a handover process according to an embodimentof the present invention.

Referring to FIGS. 8 and 9, a UE performs communication using CCsallocated thereto by a serving cell in a state in which a handover isnot triggered (S802 and S804). For convenience of explanation, the CCsallocated to the UE are referred to as an active CC set. The CCs may beallocated according to a UE-specific, UE-common or cell-specific method.For example, the CCs can be allocated using RRC signaling and/or L1/L2control signaling. Component carrier allocation through UE-specific orUE-common L1/L2 control signaling may be performed dynamically orsemi-dynamically. Component carrier allocation using L1/L2 controlsignaling can override component carriers allocated through RRCsignaling.

When a handover condition is satisfied (for example, when signalintensity is decreased), the UE performs neighbor cell measurement(S806), and reports the measurement result to the serving cell (S902). Abase station of the serving cell determines the handover on the basis ofthe measurement report of the UE (S808), and transmits a handoverrequest message with UE context to a target cell (S904). The target celltransmits a handover request response message to the serving cell(S906). The handover request response message includes basic informationrequired for the UE to perform initial transmission in the target cell.Specifically, the handover request response message includes new C-RNTI,part of a handover command message, and information on a dedicatedsignature for non-contention based random access (that is, random accesspreamble). The signature is reserved at this time. Furthermore, thehandover request response message includes information on componentcarriers of the target cell. The information on the component carriersincludes DL CC and/or UL CC setup information for performing initialaccess and/or initial transmission and reception in the target cell, orDL CC and/or UL CC configuration information constituting the cell.

The serving cell transmits the handover command message to the UE (S810and S908). The handover command message includes new C-RNTI, randomaccess related information (for example, a signature route sequenceindex, a cyclic shift parameter, and dedicated signature), andinformation on CCs of the target cell. The UE acquires basic informationrequired to perform initial transmission in the target cell from thehandover command message, and then transmits a random access preamble(for example, a dedicated random access preamble) to the target cell(S812 and S910). Subsequently, the UE synchronizes with the target cellthrough message hand shaking and performs an information exchangeprocess, to thereby complete the handover process (S912, S914 and S916).Then, the UE can receive UE-specific component carrier information fromthe target cell through RRC signaling and/or L1/L2 control signaling(S814).

Processes which can be related with carrier aggregation setup among UEoperations in the handover process will be explained in more detail.

Process 1: The UE Measures a Neighbor Cell (S806 of FIG. 8).

The UE may have two CC allocation schemes at a handover triggering time.

In the first scheme, the UE can be converted into a single CC state on adownlink and/or an uplink at the handover triggering time inconsideration of the handover triggering time and probability of asituation in which geometry on a channel of the corresponding UE isdeteriorated. Conversion into the single CC state can be performedthrough RRC signaling or L1/L2 control signaling. That is, in a handovertriggering state, the UE may be in a state in which carrier aggregationis not set up (scheme A). Scheme A is identical to the neighbor cellsearch method in the LTE and means that intra-frequency measurement ispreferentially performed through one DL CC.

In the second scheme, a specific state may not be premised for carrieraggregation of the UE at the handover triggering time considering thatallocation of CCs to the UE is determined by RRC or scheduler. That is,UE CC allocation may be performed in a carrier aggregation state or in anon-carrier aggregation state at the handover triggering time (schemeB). In this state, neighbor cell measurement for the handover may beperformed on all the DL CCs (that is, active DL CC set) allocated to theUE, simultaneously. Considering that the UE can support a plurality ofDL CCs, neighbor cell measurement can be performed on all DL CCscurrently assigned through CC allocation or constituted by the basestation, or a plurality of DL CCs predetermined by signaling orarbitrary agreement from the beginning.

Alternatively, when the UE sets up a plurality of DL CCs in scheme B,neighbor cell measurement can be preferentially performed on only one orsome of the corresponding DL CCs (that is, active DL CC set). Theneighbor cell measurement can be performed on the corresponding DL CC(s)only. If an appropriate cell cannot found only with the corresponding DLCC(s), a process of searching another CC band may follow the neighborcell measurement process. CCs on which the neighbor cell measurement ispreferentially performed may be UE-specific, cell-specific orcell-common DL CC(s) for a handover, and can be referred to as primaryDL CC(s) or anchor DL CC(s) for convenience. In this case, one or moreprimary or anchor DL CCs may be commonly set for cells in apredetermined range in order to construct a stabilized handovermeasurement environment in a multi-cell environment. Definition of aprimary CC or an anchor CC described in the present invention is needed.The primary or anchor UL CC means an UL CC configured and designated foruplink control information transmission basically. In an extended sense,the primary or anchor UL CC may include an UL CC which becomes astandard for uplink physical signal transmission. The primary or anchorDL CC is defined as a specific DL CC for transmitting controlinformation that manages connection with a corresponding UE, which is aCC that is a default of CC reconfiguration. Also, the primary or anchorDL CC can be defined as a DL CC that transmits NAS (Non-Access Stratum)information for authentication and security to a UE. Furthermore, theprimary or anchor DL CC may be defined as a DL CC that transmitsspecific cell-specific, UE-common control information or UE-specificcontrol information.

Process 2: The UE Transmits Measurement Report to the Serving Cell (902of FIG. 9)

The UE can transmit a measurement report for each CC through an UL CClinked to a DL CC on which neighbor cell measurement has been performed.Here, signaling for measurement triggering may be performed through oneor more designated DL CC(s) (for example, primary or anchor CC(s)) orrespectively performed for individual DL CCs when a plurality of DL CCsare present. In addition, the measurement triggering may beindependently set for the respective DL CCs. Alternatively, when aUE-specific or cell-specific primary or anchor DL CC is set, the UE cantransmit a measurement report to an UL CC linked to the DL CC.Furthermore, when a UE-specific or cell-specific primary or anchor UL CCis set, the UE may transmit a measurement report to the UL CC.Alternatively, one or more UL CC(s) for measurement report may besignaled explicitly or implicitly, or implicitly designated by the UEwithout additional signaling. If a plurality of candidate UL CCs fortransmitting measurement report is present, one or some of the candidateUL CCs may be separately designated. Here, signaling (for example, RRCsignaling or L1/L2 control signaling) to the UE may be defined in orderto designate the UL CCs. Otherwise, the UL CCs may be designated(indirectly or implicitly) according to circumstances without signalingdefinition such that additional signaling overhead is not generated.Measurement information on the plurality of DL CCs may be joint-coded,separated or repeated. The measurement report is transmitted from the UEon a report time determined by the base station, and can be transmittedusing one or more uplink subframes.

When the serving cell determines the handover based on the measurementreport of the UE, and then transmits the handover request message to thetarget cell, the serving cell can send DL CC and/or UL CC configurationinformation and/or specific primary or anchor DL CC and/or UL CCconfiguration information on the UE together with the handover requestmessage, and thus share the CC setup state of the UE during the handoverwith the target cell. In this case, the target cell can allocate one ormore UL CCs for initial access or initial transmission and reception tothe UE on the basis of information from the serving cell. For example,the target cell can designate a required number of (dedicated) PRACHpreamble resources for the UE and feed them back to the serving cell soas to allow the UE to transmit dedicated PRACH preamble(s) through aplurality of UL CCs in the following process.

Process 3: The Serving Cell Transmits a Handover Command to the UE (S810of FIG. 8 and S908 of FIG. 9).

The base station of the serving cell generates handover commandinformation on the basis of the handover related information receivedfrom the target cell, transmits the handover command information to theUE, and starts to cancel connections of the serving cell, which relateto the UE, from an upper layer. Here, the target cell can allocate oneor more DL CCs and/or one or more UL CCs in order to provide continuityof service quality for the handover UE, and information on the DL CCsand/or UL CCs can be included in the handover command message. CCallocation information of the target cell can be used for initial accessto the target cell and/or subsequent initial transmission and reception.For this, the handover command message can directly include information(for example, index) for indicating one or more DL CCs and/or one ormore UL CCs.

The handover command message may include additional information relatedto CC allocation of the target cell if required. The additionalinformation may include UL CC configuration information for RPACHpreamble transmission and PRACH preamble resource information. Forexample, if a plurality of UL CCs is allocated and a (dedicated) PRACHpreamble is transmitted through one of the UL CCs, the additionalinformation can include UL CC index information for PRACH preambletransmission and (dedicated) PRACH preamble resource information. Inaddition, when the (dedicated) PRACH preamble is transmitted for each ULCC or for the plurality of UL CCs (for example, in case ofnon-continuous CC allocation), the additional information may includeinformation on a plurality of (dedicated) PRACH preamble resources, andUL CC index information related to the information if required. The ULCC index information and (dedicated) PRACH preamble resource informationmay be contained in the handover command message as one informationitem. The PRACH preamble resource information includes a signature routesequence index, a cyclic shift parameter, and a dedicated signature. Asequence for the signature includes CAZAC sequence and Zadoff-chusequence.

One or more UL CCs of the target cell may be set up using the handovercommand message and/or UL CC information set by the existing servingcell. The number/target of UL CCs that transmit the dedicated PRACHpreamble sequence may be embodied through a separate UL (or UL/DL) CCsetup process related to RACH. Furthermore, a limited number (includingone)/target of CCs among UL CCs set up in the target cell canindividually transmit uplink dedicated PRACH preamble without additionalCC configuration for RACH.

The UL CC configuration information can be explicitly signaled (forexample, index) through the handover command message or indirectlyconfirmed using DL CCs configuration information. For example, only DLCCs are explicitly indicated through the handover command message, andthe UE grasps information on UL CCs on the basis of CC information ofthe serving cell or information implicitly acquired from the handovercommand message. In addition, the UL CCs may be UL CCs linked to DL CCsset up by the handover command message (this linkage relationship may beset in a previous serving cell or may be common for a plurality ofcells). Furthermore, the UL CCs may relate to primary or anchor DL CCsor may be independently configured primary or anchor UL CCs according tocircumstances.

Since access to the target cell is performed upon reception of thehandover command, the handover command is not required to transmit thewhole information about UE-specific multi-CC allocation. Accordingly,the handover command can transmit only single DL/UL CC allocationinformation for accessing the target cell. Alternatively, the handovercommand may transmit information on a single DL CC, and the UE mayreceive system information from the corresponding DL CC of the targetcell and then acquire information on an UL CC linked to the DL CC. Thesingle DL CC allocated through the handover command may be a primary oranchor DL CC of the target cell. Cell-specific CC configurationinformation of the target cell may be indicated through the handovercommand.

The DL CC for initial transmission and reception between the target celland the UE during the handover may not be additionally designated by thehandover command message. In this case, the DL CC for initialtransmission and reception may be a DL CC on which measurement has beenperformed for the corresponding cell or DL CCs transmitting the handovercommand message or one of the DL CCs. Otherwise, the DL CC for initialtransmission and reception may be a series of primary or anchor DL CCsset up in the serving cell.

The handover command may be configured to induce an operation on asingle DL CC and a single UL CC in the following target cell operationin order to minimize a variation in the LTE handover process and reduceoverhead of the handover process. To achieve this, the handover commandmessage can be composed of the same information as that of the LTE.Furthermore, the handover command may include an index of a single DL CCand/or UL CC in a multi-DL/UL CC state. In particular, the handovercommand message may include index information on the corresponding ULCC. Alternatively, the UL CC may be an UL CC for which a linkage with aDL CC set up through the handover command message is set. Here, thelinkage may be a linkage set in the serving cell or a specific linkagecommonly set among a plurality of arbitrary cells. Alternatively, the ULCC may be an UL CC linked to arbitrary primary or anchor DL CC(s) andmay be one or more primary or anchor UL CCs separately configured.

Meantime, a plurality of DL CCs and/or UL CCs may be set up for thetarget cell initial operation without an additional instruction by thehandover command message. In this case, the plurality of DL CCs and/orUL CCs can be set up using a setup state of a plurality of DL CCs and/orUL CCs of the serving cell or a setup state of a plurality of primary DLCCs and/or primary UL CCs.

In view of DL CCs transmitting the handover command message in theserving cell, a base method is to transmit the handover command messageusing all one or more DL CCs configured for the corresponding UE at thecorresponding time. This may satisfy reliability of the handover commandmessage transmission reliability. Another method is to transmit thehandover command message through a primary or anchor DL CC, or aplurality of primary or anchor DL CCs. The primary or anchor DL CC maybe set up UE-specifically, cell-specifically or cell-commonly for thehandover. If the handover command message is transmitted through aplurality of DL CCs, though the DL CCs have the same transmission timingbasically, different transmission timings may be set for the respectiveDL CCs at a subframe level in consideration of processing load. Forexample, staggering or fixed offset can be applied to the transmissiontiming for each DL CC. On the other hand, the handover command messagemay be transmitted using only one DL CC (for example, a primary (anchor)DL CC or a DL CC on which neighbor cell measurement is performed) inorder to reduce processing load in the handover process or for the samepurpose as that of the LTE. Additionally, the handover command messagemay be transmitted through the overall or some of the DL CCs set up asthe active CC set for reliability of the handover command. The handovercommand message may include information on setup of one or more DL CCsand/or one or more UL CCs for supporting an operation of accessing thetarget cell. At this time, the DL CCs may be primary DL CCs of thetarget cell. In the same manner, the UL CCs may be primary UL CCs of thetarget cell. The handover command message may include information fordesignating primary DL CCs and/or primary UL CCs having the attributedescribed in the above explanation, separately from information on setupof DL CCs and UL CCs used for transmission and reception between thetarget cell and the UE in some cases.

Process 4: The UE Transmits a Random Access Preamble to the Target BaseStation (S812 of FIG. 8 and S910 of FIG. 9).

The UE can transmit a (dedicated) PRACH preamble through one or more ULCCs of the target cell using information about a (dedicated) randomaccess preamble allocated through the handover command message.Information (for example, the number and/or indexes of UL CCs) on the ULCCs which transmit the (dedicated) PRACH preamble may be embodiedthrough a process of setting up UL CCs or DL CCs/UL CCs in the targetcell during the handover process, and additional information (forexample, the number and/or indexes of UL CCs) on setup of the UL CCs fortransmitting the (dedicated) PRACH preamble may be signaled to the UE.For example, the UE can transmit the (dedicated) PRACH preamble througha designated UL CC after receiving UL CC index information fortransmitting PRACH and (dedicated) PRACH preamble resource informationthrough the handover command message.

When the (dedicated) PRACH preamble is transmitted through one or moreUL CCs designated in the target cell, the UE can confirm UL CCinformation (for example, index) about the transmission of the(dedicated) PRACH preamble through the UL CCs using the handover commandmessage. For example, the UL CC information can be directly included inthe handover command message, or configured by the UE as detailed PRACHpreamble transmission resource information in connection with the(dedicated) RACH preamble resource information transmitted to the UEthrough the handover command message. Alternatively, UL CCs can be setup in the target cell on the basis of information on setup of CCs forthe UE and setup of one or more primary CCs from the serving cell at thehandover time, as described above, without signaling of the detailed ULCC information from the target cell. To achieve this, advanceinformation sharing between the serving cell and the target cell may bepremised. Alternatively, the target cell can blind-detect a (dedicated)PRACH preamble signal of the UE for the UL CCs configured by the targetcell.

A detailed method for the above-mentioned advance information sharingbetween the serving cell and the target cell is explained. When theserving cell determines the handover based on the measurement reportfrom the UE and then transmits the handover request message to thetarget cell, the serving cell can send DL CC/UL CC configurationinformation and/or specific primary or anchor DL/UL CC configurationinformation for the UE at the time when the handover request message istransmitted from the serving cell together with the handover requestmessage such that the target cell shares a carrier setup state in theserving cell for the UE during the handover. In addition, when the UEtransmits the (dedicated) PRACH preamble through one or more UL CCs, thetarget cell can designate a required amount of (dedicated) PRACHpreamble resources on the basis of the information shared with theserving cell, and feed the designated (dedicated) PRACH preambleresources back to the serving cell. Upon reception of the (dedicated)PRACH preamble resources, the serving cell can transmit the (dedicated)PRACH preamble resources to the UE through the handover command message.

UL CCs transmitted by the UE can be set up in the target cell on thebasis of information on setup of CCs for the UE and information on setupof one or more primary CCs from the serving cell at the handover timewithout signaling of the UL CCs. In this case, UL CC configurationinformation can be shared in advance between the target cell and theserving cell. The target cell can detect the dedicated PRACH preamble ofthe UE through blind detection for each UL CC without sharing additionalinformation with the serving cell.

Process 5: The UE Performs Synchronization with the Target Cell and theHandover Process is Completed (S912 and S914 of FIG. 9).

If the handover through a non-contention based RA process (using adedicated RA preamble) fails, the UE performs synchronization with thetarget cell through a contention based RA process. Upon completion ofthe handover with the target cell, the target cell transmits a handovercomplete message to the serving cell. The serving cell receives thehandover complete message, completes all connections of user/controlplanes with the UE, and completes connections for respective CCs.

Process 6: The Target Cell Sets Up DL CCs and/or UL CCs for the HandoverUE (S814 of FIG. 8).

Upon completion of the handover process, the UE can transmit/receivedata/control information to/from the target cell through DL/UL CC(s)allocated in the handover process (for example, handover commandmessage, RA process). The target cell can signal DL/UL CC allocationinformation through UE-specific (or cell-specific) RRC signaling and/orUE-specific (or cell-specific) L1/L2 control signaling in order toupdate CC allocation to the UE. For example, the CC allocationinformation can be included in a response message for transmission ofone or more (dedicated) RACHs, and can be separately signaled in aresponse message transmission process or the following process.

Methods for configuring, setting up and signaling DL/UL CCs on ahandover complete time in the target cell can be implemented in the samemanner as allocation of CCs to the UE in the serving cell before thehandover process. For example, the CC allocation information candirectly include information (for example, index) that indicates one ormore DL CCs and/or one or more UL CCs. UL CC configuration informationmay be explicitly signaled, or indirectly confirmed using DL CCconfiguration information. For example, UL CC information can beindirectly confirmed using DL CC and UL CC linkage relationship.

The proposed methods relating to configuration, setup and signaling ofDL CCs and/or UL CCs for the UE before and after the handover process inthe above processes 1 to 6 can be individually performed, or the overallor some of the methods can be combined and carried out.

Embodiment 1-2: Handover 2 in Consideration of Frequency Aggregation

The embodiments 1-1 and 1-2 can be used together or independently used.The fundamental concept of the conventional handover process is that aUE receives a handover command, and then leaves a serving cell. However,the UE can perform a handover only for some (for example, one) of aplurality CCs in the embodiment of the present invention. In otherwords, if the UE uses two DL CCs and two UL CCs and can handle at mosttwo CC, some of DL/UL CCs can perform a handover and the remaining DL/ULCCs can transmit/receive traffic with the serving cell. The DL/UL CCsperforming the handover may be primary CCs or anchor CCs. When the UEsucceeds in connecting with the target cell, the UE can transmit/receivedata to/from the target cell through a DL/UL CC set up by the targetcell during the handover. Then, the remaining DL/UL CCs to be used bythe UE in the target cell can be additionally set up through(UE-specific) carrier aggregation in the target cell.

To complete the handover process for the DL/UL CC set of the UE andtransmit/receive data in the target cell, a process of cancelingconnection of the remaining DL/UL CC(s) is required. To achieve this,the target cell can transmit a message (for example, a handover confirmmessage) that indicates successful completion of the handover to theserving cell. Upon reception of the handover confirm message, theserving cell can send information for canceling the connection of theremaining DL/UL CCs with the serving cell to the UE through signaling.When connection of the active DL/UL CC(s) allocated to the UE by theserving cell is released, the UE can be re-allocated UE-specific DL/ULCCs by the target cell through the DL/UL CCs used for the handover.Furthermore, upon release of connections for all the active DL/UL CC(s)allocated to the UE from the serving cell, the UE can use the DL/UL CCsused for the handover in the serving cell or CCs relating to the DL/ULCCs in the target cell even without additional signaling from the targetcell.

Although a base station allocates DL/UL CCs to a UE depending on UEcapacity, the base station does not always allocate DL/UL CCscorresponding to the UE capacity. Otherwise, there may exist CCs whichare not used though allocated to the UE. If the UE has remainingcapacity and CCs unused in the serving cell are present, the UE canperform a handover using DL/UL CCs corresponding to the remainingcapacity thereof. That is, the UE can receive and transmit a downlinkphysical signal/physical channel and an uplink physical signal/physicalchannel, which are required for the handover, respectively using thecorresponding DL/UL CCs. If a primary DL CC and/or a primary UL CC isdefined among active CCs allocated to the UE, the UE can access thetarget cell using the corresponding primary CC. If primary DL CCs orprimary UL CCs are aggregated in the serving cell, the base station canselect the corresponding primary CC as a CC for the handover.

Embodiment 2: Handover in Consideration of CoMP

The embodiment 1 is based on the assumption that the handover process ofthe UE is identical to the handover process defined in the LTE. However,when coordinated multiple points (CoMP) transmission/reception or asoftware handover (a kind of CoMP), which is newly considered in anadvanced system such as LTE-A, is introduced, a downlink or uplink CoMPsituation may be premised during a handover preparation process orbefore the handover preparation process. Additional suggestions for ahandover process including CoMP and changed suggestions will now beexplained. For convenience, CoMP is used as a concept including thesoftware handover.

Two options may be considered for a handover processing including CoMP.Option 1 corresponds to a case in which a CoMP transmission mode is setbefore the handover preparation process. Option 2 corresponds to a casein which the CoMP transmission mode is set after the handoverpreparation process is triggered. For example, in case of option 2, adownlink and/or uplink CoMP mode can be set during the handoverpreparation process. Specifically, the CoMP mode can be set before orafter the neighbor cell measurement process. When the CoMP mode is set,connection with the serving cell can be finished at an arbitrary time onthe basis of RSRP (Reference Signal Received Power) and/or RSRQ(Reference Signal Received Quality) of the serving cell.

An embodiment of the present invention is described on the basis ofoption 1. In option 2, the process described in the embodiment 1 can beapplied to a handover process before the CoMP mode is set. On the otherhands, suggestions of option 1 which will be described below can beapplied to handover processes for a corresponding UE after the CoMP modeis set.

Option 1 is based upon a situation in which the UE is connected with oneor more neighbor cells before a handover. Accordingly, neighbor cellmeasurement in the handover preparation process can be replaced bymeasurement for a measurement cell set in the CoMP mode on the basis ofRSRP or RSRQ Of the serving cell. For CoMP measurement, particulars ofneighbor cell measurement, illustrated in S806 of FIG. 8 can be appliedto a method for configuring, setting up and signaling DL CCs for thecorresponding UE. Alternatively, the neighbor cell measurement processfor the handover can be performed separately from CoMP measurement onthe basis of a difference between a target cell of CoMP measurement anda target cell of neighbor cell measurement, or attributes of themeasurement. The neighbor cell measurement can employ the neighbor cellmeasurement process illustrated in S806 of FIG. 8.

In the same manner, a measurement report process for the handover can bereplaced by a process of reporting a measurement result with respect toa reporting cell set in the CoMP mode. In this case, the particularsdescribed in the process 2 (S902 of FIG. 9) of the embodiment 1 can beapplied to a method for configuring, setting up and signaling DL CCsand/or UL CCs for the corresponding UE for CoMP measurement report.Alternatively, the measurement report process for the handover can bedefined separately from the CoMP measurement report process on the basisof a difference between CoMP and the handover in a target reporting cellor difference between reporting measurement attributes. In this case,the CoMP measurement report process can employ the method forconfiguring, setting up and signaling CCs, described in the process 2(S902 of FIG. 9) of the embodiment 1.

As the CoMP mode is set for an arbitrary UE, one of one or more cells inan active cell set on DL CoMP or UL CoMP, which is directly orindirectly involved in transmission and reception, can be set as atarget cell in a handover determination process of the serving cell. Inthis case, downlink transmission of a handover command message,transmission of a (dedicated) PRACH preamble sequence between the targetcell and the corresponding UE, and response processes for thetransmissions can be eliminated. Furthermore, a CC allocation process(for example, the process 6 of the embodiment 1) for the correspondingUE, which is attended in a frequency aggregation state, can beeliminated. As described above, in a state in which part of theprocesses exemplified in the embodiment 1 is eliminated, the servingcell can inform the UE that connection with the serving cell is endedthrough additional RRC signaling or L1/L2 signaling (for example,completion indicator.

When a plurality of DL CCs are set up between the serving cell and theUE, the method of transmitting the handover command message in theembodiment 1 can be applied to a DL CC transmitting the completionindicator. Specifically, upon determination of a handover, the servingcell can transmit a handover request message to the target cell, and thetarget cell can send a response to the handover request message to theserving cell. In this case, when the serving cell indicates thatconnection with the serving cell is ended to the corresponding UEthrough additional RRC signaling or L1/L2 signaling, the UE canrecognize the target cell as a serving cell and/or an anchor cell onCoMP right after the indication or from a designated time. A messagethat indicates that the connection with the serving cell is endedthrough RRC signaling can be defined in a new message format for LTE-AUEs rather than a handover command message format that is not based onCoMP. When a plurality of DL CCs are set up between the serving cell andthe UE, an arbitrary method from among the handover command messagetransmission methods proposed in the embodiment 1 can be applied.

Alternatively, only the downlink CoMP transmission mode is applied to anarbitrary UE (or only a downlink CoMP active cell set is defined anduplink CoMP operates as UE transparent CoMP), and an arbitrary cell in adownlink active set can be set as a target cell in the handoverdetermination process of the serving cell. At this time, the handovercommand transmission method and the subsequent processes described inthe embodiment 1, and the suggestions with respect to DL CC and UL CCsetup can be applied as processes following the handover determinationprocess. In a different manner, a modified handover command message fromwhich information on downlink connection has been eliminated can beconsidered since downlink transmission connection of the target cell isset up. The modified handover command message may be replaced by a newmessage in a format different from the handover command message andapplied. Subsequently, a process of transmitting a (dedicated) PRACHpreamble from the UE to the target cell, a process of receiving aresponse message for the (dedicated) PRACH preamble, and a process ofsetting up DL CCs and/or UL CCs for the UE if required can be performed.In addition, at least part of target cell related information fordownlink connection setup can be eliminated from a response message totransmission of a (dedicated) PRACH preamble sequence. Theabove-mentioned method can be applied to even a case in which thedownlink CoMP mode in which the active cell set is not signaled to theUE is employed.

Alternatively, only the uplink CoMP transmission mode can be applied(including a case in which downlink CoMP operates as UE transparentCoMP. Here, an uplink CoMP active set may be set or not). The uplinkCoMP mode includes a case in which receiving synchronization is made anda case in which receiving synchronization is not made. Basically,application of the handover command message transmission method and thesubsequent processes in the embodiment 1 can be considered. If receivingsynchronization has been made between the UE and the target cell,(dedicated) PRACH preamble sequence transmission can be eliminated, andthus (dedicated) PRACH preamble resource information can be eliminatedfrom the handover command message. However, it may be desirable totransmit the (dedicated) PRACH preamble sequence for the purpose ofinducing a time when the UE is connected with the target cell and asubsequent response message.

In a state in which a CoMP mode is set and DL CC/UL CC setup isachieved, a handover can be implemented according to switching of areference cell (referred to as a downlink or uplink CoMP anchor cell forconvenience) on a downlink and/or uplink transmitting controlinformation in the CoMP mode. Processes for this implementation can bedefined as follows.

Process 1: The handover or anchor cell switching is determined on theCoMP anchor cell on the basis of a measurement report result separatelytriggered based on measurement on CoMP or a variation in RSRP or RSPQ.

Process 2: A target cell or a new anchor cell switching candidate isdirected to perform CoMP anchor cell switching, and a message includingprofile information of the corresponding UE is transmitted from aserving anchor cell to a target CoMP anchor cell (or a target cell, ananchor cell switch candidate). The profile information includesinformation required to set a new CoMP anchor cell. The profileinformation is UE-specific information and it can include DL/UL setupinformation of the corresponding UE.

Process 3: A message including information required for the UE to setthe new CoMP anchor cell is transmitted as a response to the process 2from the target CoMP anchor cell to the serving CoMP anchor cell.

Process 4: Upon reception of the message of the process 3, the servingCoMP anchor cell transmits a CoMP anchor cell switching message to theUE. The methods of configuring, setting up and signaling CCs,exemplified in the process 3 of the embodiment 1, can be applied to themessage.

Process 5: Upon reception of the message of the process 4, the UE cantransmit an additional ACK/NACK signal for confirming whether or not themessage has been correctly received through an uplink control channel oran uplink shared channel, or transmit a (dedicated) PRACH preamblesequence. Resource information for transmitting the ACK/NACK signal or(dedicated) PRACH preamble sequence can be transmitted from the targetCoMP anchor cell to the UE through the process 3 and the process 4.Subsequently, the target CoMP anchor cell transmits a message (CoMPanchor cell switching complete message) which indicates that CoMP anchorcell switching is completed to the serving CoMP anchor cell. Uponreception of the CoMP anchor cell switching complete message, theserving CoMP anchor cell cancels its anchor role and connection for theUE so as to complete the overall CoMP anchor cell switching process. If(dedicated) PRACH preamble sequence transmission is performed, aresponse to the (dedicated) PRACH preamble sequence transmission isadditionally transmitted to the UE from the target CoMP anchor cell, andthe UE may additionally transmits an ACK/NACK signal before the targetCoMP anchor cell generates the CoMP anchor cell switching completemessage upon reception of the response from the target CoMP anchor cell.Furthermore, even when the UE transmits a message representing whetheror not the message of the process 4 has been correctly received (throughan ACK/NACK signal) through the uplink control channel or uplink sharedchannel, the UE can additionally transmit a downlink ACK/NACK signal forthe message before the target CoMP anchor cell generates the CoMP anchorcell switching complete message. The above-mentioned handover process ofthe UE to which the CoMP transmission mode is applied, and DL CC/UL CCsetup and signaling schemes for the handover process are individualschemes, and arbitrary schemes among the individual schemes may beapplied in a combined form.

Embodiment 3: Detailed Carrier Aggregation Schemes Relating to aHandover

The following three elements can be considered as carrier aggregationschemes which can be considered in a handover process.

Element 1: Introduction of Extension CC or Frequency Resource Segment

A CC that can perform fundamental connection, cell search and systeminformation transmission processes for a UE on an arbitrary cell, basestation or relay node alone can be defined as a stand-alone CC throughdefinition of a physical channel and a physical signal in the same formas LTE Rel-8 carrier. A CC having non-stand-alone property which doesnot support the above-mentioned processes, different from the propertyof the stand-alone CC, can be defined as an extension CC. The extensionCC does not transmit PSS (Primary Synchronization Signal)/SSS (Secondarysynchronization Signal)/P-BCH, and may not transmit a DBCH (DynamicBroadcast Channel) transferring system information and common PDCCH forthe DBCH. In addition, the extension CC may not transmit a DL channelallocation PDCCH and UL grant PDCCH in a format defined at least in theRel-8 LTE. Accordingly, it is not necessary to set a transmission regionof PDCCH defined in the LTE Rel-8 and a CRS (Cell-specific ReferenceSignal set up in the LTE Rel-8), and as a CFI (Control Format Indicator)indicating them is not needed, there is no need to transmit a PCFICH(Physical Control Format Indicator Channel) defined in the LTE Rel-8. Itcan be considered that the extension CC has a bandwidth corresponding toscalable BW of the LTE Rel-8. However, the bandwidth of the extension CCcan be defined as a bandwidth different from the scalable BW based onpurposes such as utilization of specific residual resources. Similarly,as a specific frequency resource region in a CC, a resource region fromwhich the overall particulars described with respect to the extensionCC, and some physical channels or physical signals are excluded can bedefined as a segment.

Element 2: Cross-CC Scheduling for Carrier Aggregation

When a PDSCH is transmitted from a cell (or a relay node as atransmission subject) to a UE (or a relay node as a receiving subject)through an arbitrary DL CC, a PDCCH (that is, DL channel allocationPDCCH) for scheduling the PDSCH can be transmitted through a DL CCdifferent from the DL CC transmitting the PDSCH. This is defined asdownlink cross-CC scheduling or downlink cross-carrier scheduling.Furthermore, when a PUSCH is transmitted from a UE (or a relay node as areceiving subject) to a cell (or a relay node as a transmission subject)through an arbitrary UL CC, a PDCCH (that is, UL grant PDCCH) forscheduling the PUSCH can be received through a DL CC which is not linkedwith an UL CC transmitting a PUCCH. This is defined as uplink cross-CCscheduling. One or more specific DL CC(s) for transmitting the DLchannel allocation or UL grant PDCCH can be set cell-specifically orUE-specifically. This specific DL CC(s) can be referred to as a primaryDL CC or anchor DL CC. In addition, one or more specific DL CC(s) fortransmitting the DL channel allocation or UL grant PDCCH can be set as aUE-specific or cell (or relay node)-specific PDCCH monitoring CC set.

Element 3: Dynamic/Semi-Static CC Activation/Deactivation

Update of a DL/UL active CC set which is UE (or relay node as areceiving subject)-specifically set, or activation/deactivation of DLCC(s) or UL CC(s) set as the DL/UL active CC set can be dynamicallyinstructed, or whether or not to perform the activation can besemi-statically set using cell (or relay node as a transmissionsubject)-specific or UE (or relay node as a receiving subject)-specificRRC signaling.

Detailed schemes of a handover process based on the above schemes aresuggested as follows.

Handover Process and Operation in Consideration of Introduction ofExtension CC or Segment

From the standpoint of neighbor cell measurement and report beforehandover triggering, LTE Rel-8/9 UEs or Rel-10 LTE-A UEs may not performneighbor cell measurement for an arbitrary extension CC. Furthermore,the extension CC may have different configurations for respective cells(or relay nodes as transmission subjects). In view of this, thefollowing various schemes can be applied.

Scheme 1: The scheme A of the embodiment 1 is assumed. That is, assuminga case in which carrier aggregation is not applied (single DL CC-UL CC)during neighbor cell measurement for a handover. On the assumption thata serving DL CC of a serving cell is an extension DL CC of a neighborcell, when high priority is set for intra-frequency measurement as inthe LTE Rel-8 system, handover triggering to an optimum cell or FA(Frequency Assignment) (CC) may not be performed. When a situation inwhich an extension CC is cell-specifically configured is assumed,coordination or a special operation for CC configuration can be definedin order to effectively perform neighbor cell measurement. For example,active camping is performed to a specific DL CC configured by theserving cell before neighbor cell measurement is executed for ahandover. Furthermore, when a single CC is allocated, a method ofallocating a specific DL CC that is cell (or relay node as atransmission subject)-specifically set can be applied. In this case, thecorresponding DL CC is a DL CC which is not configured as an extensionCC in a neighbor cell (or relay node as a transmission subject) and itcan be coordinated in advance when DL CCs are configured. According tothis scheme, cell boundary UEs may be concentrated on a specific DL CCso as to bring about high interference. To solve this problem, aplurality of cell (or relay node as a transmission subject)-specificallyset DL CCs can be defined. In addition, active camping is performed to aspecific DL CC before neighbor cell measurement for the handover iscarried out, or a DL CC can be UE (or relay node as a downlink receivingsubject)-specifically set when a single CC is allocated. Here, thecorresponding DL CC(s) is DL CC(s) which is not configured as anextension CC in a neighbor cell (or relay node as a transmissionsubject) and can be coordinated in advance when DL CCs are configured.Alternatively, a neighbor cell can inform the UE of information onextension CC configuration prior to neighbor cell measurement. On thecontrary, the UE can be informed of information (for example, CC indexinformation) on configuration of measurable DL CCs which are notextension CCs. The CC index information in these two cases can beincluded in a neighbor cell list as a parameter or defined as anadditional parameter, and broadcasted to UEs through the serving cell.This is a method of enabling a network operator to freely configure CCsfor each cell and can reduce latency or complexity/expenses of the UE inneighbor cell measurement.

Scheme 2: The scheme B of the embodiment 1 is assumed. That is, it isassumed that carrier aggregation (multi-DL CC setup) is applied toneighbor cell measurement for a handover. When a case in which theoverall or some of serving DL CCs of the serving cell are extension CCsof a neighbor cell is assumed, all set DL CCs can be defined as anintra-frequency measurement target in the neighbor cell measurement, andintra-frequency neighbor cell measurement can be performed through a DLCC specifically set or set to a primary or anchor CC. When high priorityis set for intra-frequency measurement as in the LTE Rel-8 system,handover triggering to an optimum cell or FA (Frequency Assignment) (CC)may be not performed. On the assumption that an extension CC iscell-specifically configured, coordination or a special operation on CCconfiguration for enabling the corresponding UE to effectively performneighbor cell measurement can be defined. For example, when activecamping is performed to a specific DL CC or a single CC is allocatedbefore neighbor cell measurement for a handover is performed in a statethat a plurality of DL CCs are assigned (active DL CC set is assigned)to a UE, a method of allocating a specific DL CC which is cell (or relaynode as a transmission subject)-specifically set can be applied. Thespecific DL CC can be set by updating the active DL CC set through UEdedicated RRC signaling or PDCCH. In addition, the specific DL CC may beset by leaving only the specific DL CC in an active state anddeactivating other DL CCs. In this case, the corresponding DL CC is a DLCC which is not configured as an extension CC in a neighbor cell (orrelay node as a transmission subject) and can be coordinated when DL CCsare configured.

According to the present invention, cell boundary UEs may beconcentrated on a specific DL CC so as to bring about high interference.To solve this problem, a plurality of cell (or relay node as atransmission subject)-specifically set DL CCs can be defined. Inaddition, active camping is performed to a specific DL CC beforeneighbor cell measurement for the handover is carried out, or a DL CCcan be UE (or relay node as a downlink receiving subject)-specificallyset when a single CC is allocated. Here, the corresponding DL CC(s) isDL CC(s) which is not configured as an extension CC in a neighbor cell(or relay node as a transmission subject) and can be coordinated inadvance when DL CCs are configured. Alternatively, a neighbor cell caninform the UE of information on extension CC configuration prior toneighbor cell measurement. On the contrary, the UE can be informed ofinformation (for example, CC index information) on configuration ofmeasurable DL CCs which are not extension CCs. The CC index informationcan be included in a neighbor cell list as a parameter or defined as anadditional parameter, and broadcasted to UEs through the serving cell.This is a method of enabling a network operator to freely configure CCsfor each cell and can reduce latency or complexity/expenses of the UE inneighbor cell measurement.

Meantime, a handover command transmission may employ an arbitrary methodamong the above-mentioned methods. When a handover message is repeatedlytransmitted through multiple DL CCs, a method of excluding extension CCsand setting the corresponding DL CCs can be considered as an additionalmethod. In addition, it is possible to consider a method of applying nocross-CC scheduling to handover command message transmission regardlessof whether or not the cross-CC scheduling is set. Alternatively,information on an extension CC configured by the target cell can beindicated as a parameter recognizable by the UE in a message (forexample, handover command) which represents DL CC configurationinformation on the target cell. Furthermore, if DL CCs assigned by thetarget cell to the handover UE includes an extension CC, the extensionCC may be indicated as a parameter recognizable by the UE in thehandover command message. Another method sets extension CCs for handoverUEs through UE (or relay node as a receiving subject)-specific RRCsignaling after the overall handover process is completed, and sets DLCCs for a handover command to back-support DL CCs or non-back-support DLCCs as stand-alone DL CCs different from extension CCs.

Handover Process and Operation in Consideration of Cross-CC Scheduling

In a case in which frequency aggregation (that is, a plurality of DLCCs) is set up by a serving cell or a relay node as a downlinktransmission subject and applied during a handover process, cross-CCscheduling can be applied when a handover related message (for example,a handover command message, or a response message to (dedicated) PRACHpreamble transmission) is downlink-transmitted. In this case, whetherthe handover related message will be transmitted through one DL CC orthrough a plurality of DL CCs needs to be considered. In view of this,detailed methods for downlink-transmitting the handover related messageare suggested as follows.

Whether or not to Apply Cross-CC Scheduling to the Handover RelatedMessage

Method 1: Cross-CC scheduling is deactivated at an arbitrary time priorto transmission of the handover related message, and related DL channelallocation PDCCH is transmitted through a DL CC transmitting PDSCH ofthe handover command message. In this case, the handover command messagecan be transmitted using a specially set DL CC (for example, a primaryor anchor CC) when a plurality of DL CCs are set up. If the handovercommand message is transmitted through a plurality of DL CCs, DL channelallocation PDCCHs corresponding to a number of PDSCHs need to begenerated and transmitted. In this case, the DL channel allocationPDCCHs can be transmitted through the DL CCs transmitting the PDSCHs.Otherwise, DL channel allocation PDCCHs for all the PDSCHs can betransmitted through a specially designated DL CC (for example, a primaryCC, or anchor CC). Desirably, one DL channel allocation PDCCH may betransmitted for handover command message (PDSCH) transmission through aplurality of PDSCHs on a plurality of DL CCs in order to avoid PDCCHtransmission overhead when the handover command message is transmittedthrough the plurality of DL CCs. In this case, a DL CC transmitting theDL channel allocation PDCCH may be a specially designated DL CC (forexample, a primary CC or an anchor CC).

Method 2: A state in which cross-CC scheduling is continuously activatedat a handover command message transmission time can be assumed. In otherwords, a situation in which a CC indication field is set in a DCI(Downlink Channel Information) format of DL channel allocation PDCCHsfor transmission of all PDSCHs including handover command messagetransmission can be assumed. In this case, it can be considered which DLCC is used to transmit the handover command message. The handovercommand message can be transmitted through a specific DL CC (forexample, a primary CC, or an anchor CC), and the specific DL CC may be aDL CC defined as a DL CC transmitting a PDCCH. Alternatively, thespecific DL CC may be a DL CC set in a PDCCH monitoring CC set (ifdefined). Meantime, when transmission of the handover command messagethrough a plurality of DL CCs is supposed, DL channel allocation PDCCHscorresponding to a number of PDSCHs needs to be generated andtransmitted. In this case, the DL channel allocation PDCCHs can betransmitted through DL CCs transmitting the PDSCHs and DL CCs determinedaccording to a cross-CC scheduling rule, that is, a rule for indicatingtransmission CCs of related scheduling PDSCHs according to a CCindication field in the PDCCHs. Otherwise, DL channel allocation PDCCHsfor all the PDSCHs may be transmitted using a specifically designated DLCC (for example, a primary CC or an anchor CC). This case may notconform to the cross-CC scheduling rule. Preferably, one DL channelallocation PDCCH may be transmitted for transmission of a plurality ofhandover command messages (PDSCH) in order to avoid PDCCH transmissionoverhead when the handover command message is transmitted through theplurality of DL CCs. In this case, a DL CC transmitting the DL channelallocation PDCCH may be a specifically designated DL CC (for example, aprimary CC or an anchor CC).

PDCCH transmission DL CCs of DL scheduling setup and UL grant message,which are configured by the target cell to continuously support cross-CCscheduling during a handover, may be added to physical channel/physicalsignal receiving DL CC configuration information and physicalchannel/physical signal transmission UL CC configuration of the UE,contained in the handover command message by the serving cell, andtransmitted to the UE. The PDCCH transmission DL CCs may be configuredin the same manner as the manner of configuring the primary DL CCdescribed in the present invention. If the UE is well-informed of thisfact, the PDCCH transmission DL CCs can be indicated by one parametersignaling for related DL CC configuration.

Whether or not to Apply Cross-CC Scheduling to a Confirmation Messagefor RPACH Preamble Transmitted from the UE

When a plurality of DL CCs is set as an active DL CC set for the UE inthe target cell, a confirmation message can be transmitted using thesame method as the method of transmitting the handover command message.That is, when a PDSCH carrying the confirmation message is transmittedthrough one DL CC, the DL CC may be a DL CC linked with an UL CCtransmitting a dedicated PRACH preamble, or a specifically designated DLCC (for example, a primary or anchor DL CC) regardless of the UL CC.Meantime, when a plurality of PDSCHs is transmitted through a pluralityof DL CCs, cross-CC scheduling may not be applied until the handoverprocess is completed in the target cell and setup through additional RRCsignaling is made. That is, a method of transmitting a PDCCH using a DLCC transmitting a PDSCH can be applied. Alternatively, when cross-CCscheduling is activated in the target cell through the handover commandmessage, PDCCHs and related PDSCHs can be independently transmittedthrough DL CC(s) determined according to the cross-CC scheduling rule.Furthermore, a method of transmitting a confirmation message throughPDSCHs on a plurality of DL CCs may be considered. In this case, amethod for applying DL channel allocation PDCCHs can employ the detailedschemes described for handover command message transmission.

Handover Process and Operation in Consideration of Dynamic/Semi-StaticCC Activation/Deactivation

CC setup may be dynamically or semi-dynamically managed in a handoverprocess through CC activation/deactivation based on PDCCH. For example,CC activation/deactivation may be used to designate one or more DL CC(s)which become a measurement target in the active DL CC set during aneighbor cell measurement process. Furthermore, CCactivation/deactivation may be used to designate one or more DL CC(s) tobe applied to handover command message transmission. For example, whenthe active DL CC set is configured by M DL CCs, to set N(≤M) DL CCs thatbecome a (intra-frequency) neighbor measurement target or N DL CCs usedto transmit the handover command message, the remaining (M−N) DL CCs canbe deactivated.

When the target cell assigns a plurality of DL CCs to the handover UEthrough the handover command message, a case in which the target cellwants to use only one or more specific DL CCs before the handoverprocess may be generated. In this case, an explicit parameter fordeactivating the remaining DL CC(s) may be transmitted through thehandover command message. For example, the UE can recognize DL CC(s)other than a specially designated DL CC (for example, a primary oranchor CC) among DL CCs set through the handover command message to bedeactivated.

FIG. 10 is a block diagram of the UE 10. The UE 10 includes a processor(or digital signal processor) 1010, an RF module 1035, a powermanagement module 1005, an antenna 1040, a battery 1055, a display 1015,a keypad 1020, a memory 1030, a SIM card 1025 (which may be an option),a speaker 1045, and a microphone 1050.

A user can input information such as a phone number by pressing buttonsof the keypad 1020 or by a voice using the microphone 1050. Themicroprocessor 1010 may receive and process instruction information soas to execute an appropriate function such as dialing the phone number.Operation data may be extracted from the SIM (Subscriber IdentifierModule) card 1025 or the memory module 130. Furthermore, the processor1010 may display instruction and operation information on the display1015 for reference and convenience of the user.

The processor 1010 provides instruction information to the RF module1035 to start communication such as transmission of RF signals includingvoice communication data. The RF module 1035 includes a receiver and atransmitter for receiving and transmitting RF signals. The antenna 1040facilitates receiving and transmission of RF signals. When an RF signalis received, the RF module 1035 forwards and transforms the RF signal toa baseband frequency for processing by the processor 1010. The processedsignal is converted into information that can be heard or read, andoutput through the speaker 1045, for example. The processor 1010includes protocols and functions required to perform various processesdescribed in the specification.

The aforementioned embodiments are achieved by combination of elementsand features of the present invention in a predetermined manner. Each ofthe elements or features should be considered selectively unlessspecified separately. Each of the elements or features may be carriedout without being combined with other elements or features. Also, someelements and/or features may be combined with one another to constitutethe embodiments of the present invention. The order of operationsdescribed in the embodiments of the present invention may be changed.Some elements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. It is apparent that claims which are notin an explicit quotation relation can be combined to constitute anembodiment and included as a new claim according to amendment afterapplication.

Embodiments of the present invention are explained about datatransmission and reception between a base station and a user equipment.In this specification, specific operations performed by the base stationmay be carried out by an upper node of the base station according tocircumstances. In other words, it will be apparent that variousoperations performed for communication with the mobile station in thenetwork which includes a plurality of network nodes along with the basestation may be performed by the base station or network nodes other thanthe base station. The base station may be replaced with terms such asfixed station, Node B, eNode B (eNB), and Access Point (AP). Also, theuser equipment may be replaced with terms such as MS (Mobile Station),MSS (Mobile Subscriber Station), etc.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or combinationthereof. In a hardware configuration, the embodiments of the presentinvention may be implemented by one or more Application SpecificIntegrated Circuits (ASICs), Digital Signal Processors (DSPs), DigitalSignal Processing Devices (DSPDs), Programmable Logic Devices (PLDs),Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention can be implemented by a type of a module, a procedure, or afunction, which performs functions or operations described above.Software code may be stored in a memory unit and then may be executed bya processor. The memory unit may be located inside or outside theprocessor to transmit and receive data to and from the processor throughvarious means which are well known.

Those skilled in the art will appreciate that the present invention maybe embodied in other specific forms than those set forth herein withoutdeparting from the spirit and essential characteristics of the presentinvention. The above description is therefore to be construed in allaspects as illustrative and not restrictive. The scope of the inventionshould be determined by reasonable interpretation of the appended claimsand all changes coming within the equivalency range of the invention areintended to be within the scope of the invention.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a wireless communication systemwhich supports carrier aggregation. Specifically, the present inventioncan be applied to a method and an apparatus performing a handover.

The invention claimed is:
 1. A method for performing a handoverprocedure at a user equipment (UE) in a wireless mobile communicationsystem, the method comprising: transmitting, by the UE to a source basestation, a measurement report; after transmitting the measurementreport, receiving, by the UE from the source base station, a pluralityof handover command messages before the handover procedure is completed,wherein the plurality of handover command messages contain allocationinformation indicating a plurality of Downlink Component Carriers (DLCCs) and a plurality of Uplink CCs (UL CCs) assigned by a target basestation as a part of the handover procedure, wherein the plurality ofhandover command messages are received via a plurality of another DLCCs, wherein each of the plurality of handover command messages containssame allocation information, and reception timings of the plurality ofhandover command messages are staggered by a fixed offset between theplurality of another DL CCs, and wherein the plurality of DL CCs consistof a primary DL CC and one or more non-primary DL CC(s); andtransmitting, by the UE to the target base station, a random accesspreamble via a UL CC corresponding to the primary DL CC among theplurality of UL CCs.
 2. The method of claim 1, wherein each of theplurality of UL CCs corresponds to a respective DL CC.
 3. The method ofclaim 1, wherein each of the plurality of DL CCs and the plurality of ULCCs corresponds to a respective center carrier.
 4. A user equipmentcomprising: an RF (Radio Frequency) device; and a processor operativelyconnected to the RF device and configured to: transmit, to a source basestation, a measurement report, after transmitting the measurementreport, receive, from the source base station, a plurality of handovercommand messages before a handover procedure is completed, wherein theplurality of handover command messages contain allocation informationindicating a plurality of Downlink Component Carriers (DL CCs) and aplurality of Uplink CCs (UL CCs) assigned by a target base station as apart of the handover procedure, wherein the plurality of handovercommand messages are received via a plurality of another DL CCs, whereineach of the plurality of handover command messages contains sameallocation information, and reception timings of the plurality ofhandover command messages are staggered by a fixed offset between theplurality of another DL CCs, and wherein the plurality of DL CCs consistof a primary DL CC and one or more non-primary DL CC(s), and transmit,to the target base station, a random access preamble via a UL CCcorresponding to the primary DL CC among the plurality of UL CCs.
 5. Theuser equipment of claim 4, wherein each of the plurality of UL CCscorresponds to a respective DL CC.
 6. The user equipment of claim 4,wherein each of the plurality of DL CCs and the plurality of UL CCscorresponds to a respective center carrier.